Optical mixing device

ABSTRACT

An optical mixing device (10) incorporates a rectangular multimode waveguide (14), with an input region (22) and an output region (24), two square section input waveguides (26, 28), and a detector (34). The input waveguides (26, 28) are arranged to provide first and second input radiation beams respectively to the input region (22), each beam being in the form of a square waveguide fundamental mode beam. Modal dispersion along the multimode waveguide (14) produces a single maximum incident on the detector (34) when the input beams are in phase with one another, and two maxima of like magnitude located on opposite sides of the detector (34) when the input beams are in antiphase. Intermediate these two situations three maxima are produced, the amplitudes depending on phase difference. The first and second input beams may be of like frequency producing a time-independent device output. The input beams may alternatively have different frequencies. For instance the first input beam may be a local oscillator signal produced by a coherent source of stable frequency, and the second input beam may be a Doppler shifted version of an output signal from that source. The device output then provides an intermediate frequency signal. The intensity incident on the detector (34) thus varies at the difference frequency of the two inputs. Similar devices may be constructed with additional inputs and with different forms of output.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an optical mixing device.

DISCUSSION OF PRIOR ART

Optical devices for beam mixing are well known in the prior art.Beamsplitters are employed to mix two optical beams as described by A FHarvey in "Coherent Light" p1046, Wiley, London (1970). They may be usedin free space or incorporated into waveguide systems.

Optical Y-junctions of various forms are known in the prior art for theproduction of mixed beams. Various passive symmetrical Y-junctions arediscussed by Z. Weissman, A. Hardy and E. Marom in "Mode-DependentRadiation Loss in Y-Junctions and Directional Couplers", IEEE Journal ofQuantum Electronics Vol. 25, No. 6 (1989) pp. 1200-1208. AsymmetricY-junctions are discussed by K. Shirafuji and S. Kurazono in"Transmission Characteristics of Optical Asymmetric Y Junction with aGap Region", Journal of Lightwave Technology Vol 9, No 4 (1991) pp.426-429. Active Y-junctions are also known, and examples are describedby H. Sasaki and I. Anderson in "Theoretical and Experimental Studies onActive Y-junctions in Optical Waveguides", IEEE Journal of QuantumElectronics Vol. QE-14, No. 11 (1978) pp. 883-892. Each of thesereferences discusses in detail the use of Y-junctions for beamsplitting, but gives little detail on their use for beam combination ormixing. Indeed the symmetric Y-junctions, both active and passive, areinefficient splitters. Their transmission is heavily dependent on theangle of splitting; transmission is as low as 20% for splitting of a fewdegrees.

Prior art arrangements for mixing of more than two beams involvebeamsplitter or Y-junction devices used in series. The losses ofindividual devices are therefore multiplied leading to very inefficientbeam mixing.

If subsequent detection is required then the mixed beam, or beams, maybe directed to appropriate detectors. However, in addition to theirinefficiency, prior art mixing devices based on Y-junctions also sufferfrom the disadvantage of only having one output port. This redu theavailable information concerning the relative phases of the input beams.Therefore such prior art devices are of limited usefulness forapplications such as heterodyne detection where comprehensive phaseinformation is important.

SUMMARY OF THE INVENTION

It is an object of the invention to provide an alternative form ofoptical mixing device.

The present invention provides an optical mixing device for operation ata wavelength γ and including:

(a) a waveguide having an input region and an output region, and

(b) radiation supplying means arranged to provide two input radiationbeams directed to the input region,

characterised in that

(A) the waveguide is a multimode waveguide,

(B) detecting means are arranged to receive radiation transmitted by thewaveguide to the output region, and

(C) the dimensions of the waveguide, and the positions and spatialcharacteristics of the input radiation beams are in combination arrangedto provide for modal dispersion in the waveguide giving rise to inputradiation mixing in the output region and mixed radiation detection bythe detecting means.

The present invention provides the advantage that two input beams may beefficiently mixed. Theoretically the invention might provide mixing with100% efficiency. In practice, efficiencies of 75% have been achieved innon-optimised embodiments of the invention. The invention provides theadditional advantage that phase information contained in input radiationbeams is not lost when the beams are mixed. This enables mixing to becarried out prior to detection. The waveguide incorporated in devices ofthe invention may be of rectangular cross-section, of height 2a andwidth 2b, where b is greater than a. The detecting means may be locatedcentrally in the output region. Additional detecting means may beincluded. These may be two additional detecting means located within theoutput region distant b/2 to either side laterally of the centre of theoutput region, where b is as defined earlier. In such a device thewaveguide may be of length L=2b² /γ, where γ is the wavelength of inputradiation measured within the waveguide.

Mixing devices of the invention may incorporate first and seconddetecting means. Each of these detecting means may be located within theoutput region and distant b/2 laterally from and on a respective side ofthe output region centre. The waveguide may be of length L equal toeither 4b² /γ or 8b² /γ.

Mixing devices of the invention may be constructed with additional inputradiation beams.

The waveguide may be formed as a hollow within solid dielectricmaterial. The dielectric material may be alumina. Alternatively thewaveguide may be formed as a ridge waveguide upstanding from asubstrate. It may be formed of layers of a ternary or quaternarysemiconductor material system such as Al_(x) Ga_(1-x) As.

The two or more input radiation beams may be supplied by square crosssection input waveguides arranged for operation in fundamental mode.This provides spatial characteristics of the radiation electric field inthe form of a half-cycle of a sine wave.

A mixing device of the invention arranged as a heterodyne mixer may beincorporated in an optical system which includes a coherent radiationsource arranged to generate an output beam and a local oscillator beamsignal, and means for collecting radiation reflected or scattered from atest region. Such a mixing device is arranged to mix the localoscillator beam and the collected radiation.

BRIEF DISCUSSION OF THE DRAWINGS

In order that the invention may be more fully understood, embodimentsthereof will now be described, by way of example only, with reference tothe accompanying drawings, in which:

FIG. 1 is a schematic sectional plan view of an optical device of theinvention in the form of a mixer for use in heterodyne detection;

FIG. 2 is a sectional view on line II--II in FIG. 1 looking in thedirection of the arrows;

FIG. 3 graphically illustrates the variation of intensity couplingcoefficients for rectangular waveguide EH_(mn) modes with variation inthe aspect ratio of the waveguide;

FIG. 4 shows field amplitude distributions for various lower orderrectangular waveguide modes;

FIGS. 5 and 6 illustrate variation in electric field intensitydistribution as a function of position along multimode waveguides withaspect ratios of 3 and 6 respectively.

FIG. 7 provides the phase variation along each of the intensitydistributions in FIGS. 5 and 6;

FIGS. 8 and 9 illustrate variation in electric field intensitydistribution as a function of position along a multimode waveguide fortwo input radiation beams which are respectively in phase and inantiphase with one another;

FIGS. 10 and 11 graphically illustrate relative modal amplitudes of oddand even numbered waveguide modes excited in a rectangular waveguide bya fundamental mode input beam;

FIGS. 12 and 13 schematically illustrate devices of the invention foruse in heterodyne detection;

FIG. 14 schematically illustrates a laser radar system incorporating amixing device of the invention;

FIG. 15 schematically illustrates a further mixing device of theinvention.

DETAILED DISCUSSION OF PREFERRED EMBODIMENTS

Referring to FIGS. 1 and 2, there are shown sectional views of anoptical device of the invention in the form of a mixer indicatedgenerally by 10. The mixer 10 incorporates a rectangular block 12 with arectangular cross-section hole running through it to define arectangular waveguide 14. The waveguide 14 has constant rectangularcross-section and reflecting walls 16a to 16d; it is of height 2a, width2b and length L, these dimensions being respectively parallel to x, yand z Cartesian coordinate axes indicated by 18 and 20. Of these, x isreferred to as vertical (perpendicular to the plane of FIG. 1) and y andz as horizontal (in the plane of FIG. 1), for ease of expression. Theorigin of the co-ordinate system is defined, for the purpose of thisspecification, to be such that dashed line A--A in FIG. 1 indicates theplane z=0, and walls 16a to 16d lie in planes y=-b, x=+a, y=+b and x=-arespectively. The waveguide 14 has an input region 22 in the plane z=0,and an output region 24 in the plane z=L.

The parameters a, b and L are employed to preserve generality, specificvalues will be described later. However, in this example b>2a.

The mixer 10 also incorporates two square cross-section input waveguides26 and 28. The input waveguides 26, 28 have output apertures 30 and 32arranged in the input region 22 of waveguide 14 such that their centres30a and 32a are located at x=0, y=-b/2, z=0 and x=0, y=+b/2, z=0respectively. A detector 34 is arranged in the output region 24 of thewaveguide 14 such that its centre is located at x=0, y=0, z=L.

The waveguides 14, 26 and 28 are formed of alumina. The detector 34 is amercury cadmium telluride detector with associated circuitry of knownkind.

The operation of the mixer 10 will now be described in general terms; amore detailed theoretical analysis will be given later. The inputwaveguides 26 and 28 receive input radiation from a coherent source (notshown), and each carries a fundamental EH₁₁ mode radiation beam. Theseradiation beams in the input waveguides 26 and 28 provide twofundamental EH₁₁ mode inputs to the rectangular waveguide 14, in which anumber of EH_(mn) modes are excited in consequence. These modes interactwith each other as described in detail later. The effects produced bythe interaction depend on the relative frequencies and phases of theinput beams. Two input beams which are of like frequency and which arein phase with one another produce a single central maximum which iscentred at the point x=0, y=0, z=L and which is incident on the detector34. However, when two input beams of like frequency are in antiphase,the input field is regenerated with two maxima incident on respectivesides of the detector 34.

If however the two input radiation beams differ slightly in wavelength,for example by virtue of a relative Doppler shift, the electric field atthe output region 24 varies between a single central maximum and twolaterally positioned maxima. This variation occurs at the beat frequencyof the two input radiation beams. The intensity of light incident on thedetector 34 therefore varies at the beat frequency. The device 10 maytherefore be used as a heterodyne mixer. For example, one of the inputwaveguides 28 may carry a received signal obtained from a target zone byreflection or scattering of radiation an output from a coherent source;the other input waveguide 26 may then carry a local oscillator signalobtained from a like source and employed in the device 10 for mixingwith the received signal. Any beat frequency obtained by this mixingindicates Doppler frequency shift produced in the target zone frommotion of reflectors and/or scatterers of the source radiation.

The effects of mode interaction within the device 10 are the result of aspecific example of a more general phenomenon. They arise from the formof excitation of the rectangular waveguide 14 and the relationshipbetween the waveguide length L, waveguide width 2b and radiationwavelength γ. In the device 10 the length L is given by ##EQU1## where γis the wavelength of the local oscillator radiation in the rectangularwaveguide 14. The wavelength of a Doppler shifted received signal varieswith time. As will be described later, changes in the waveguide length Land in the location and form of the input to it alter the form of theelectric field at the output region.

The theoretical propagation characteristics of a rectangular waveguidewill now be analysed. It is assumed that the waveguide has height 2a,width 2b and is bounded by a homogeneous dielectric material withcomplex dielectric constant ε. It is also assumed that this dielectricmaterial (which provides the waveguide walls) is highly reflecting andnot significantly attenuating for required propagating modes. Thewaveguide has height, width and length dimensions which are parallel tothe x, y and z axes respectively. It has normalised linearly polarizedmodes of the kind EH_(mn). The electric field contribution E_(mn)(x,y,z) of the mn^(th) mode EH_(mn) at the point (x,y,z) has beencalculated by Laakmann et al in Appl. Opt. Vol. 15, No. 5, pages1334-1322, May 1976 as follows: ##EQU2## where m is the mode numberrelating to the field dependency along the x axis,

n is the mode number relating to the field dependency along the y axis,

z is the distance along the z axis,

Y_(mn) =(β_(mn) +iα_(mn)), the propagation constant of the mn^(th) mode,β_(mn) and α_(mn) being the mn^(th) mode's phase and attenuationcoefficients, and

"cos" above "sin" indicates the former applies to odd mode numbers (m orn as appropriate) and the latter to even mode numbers.

The phase coefficient β_(mn) is given by: ##EQU3##

If the negative term in parenthesis in Equation (3.1) is small comparedwith unity (paraxial radiation approximation), which is satisfied inpractice, then the binomial theorem may be used to rewrite Equation(3.1) as: ##EQU4## where a, b, m and n are as previously defined, and γis the wavelength of the radiation propagating in the waveguide.

Equation (2) sets out the electric field contributions obtainable fromall linearly polarized modes of a rectangular waveguide. It iscalculated on the basis that the electric field contribution of eachmode is zero at the side walls 16a and 16c of the waveguide, i.e. aty=+b and -b. This is satisfied if the waveguide 14 has reflecting sidewalls 16.

The first situation to be considered is that of a rectangular waveguideof side 2a by 2b excited by radiation propagating as a singlefundamental EH₁₁ ^(S) mode from a square section waveguide of side 2aconnected to one end of the rectangular waveguide and arranged coaxiallytherewith. The single EH₁₁ ^(S) mode is coupled to the various EH_(mn)modes of the rectangular waveguide. That is it becomes decomposed into alinear combination of the EH_(mn) modes with respective complexmultiplicative amplitude coupling coefficients A_(mn). For the case ofexcitation of the rectangular waveguide modes EH_(mn) by a squarewaveguide fundamental mode EH₁₁ ^(S) the coefficients A_(mn) are givenby:

    EH.sub.11.sup.S =ΣA.sub.mn ·EH.sub.mn       (4)

Essentially the A_(mn) amplitude coupling coefficients are thecoefficients of a Fourier series which represents the field at the inputregion. The EH_(mn) modes are mutually orthogonal, and in consequencethe coefficients A_(mn) can be calculated from overlap integrals of theform: ##EQU5## From Equations (2) to (5) it is possible to calculate howthe intensity coefficients I_(mn) =|A_(mn) |² of the excited rectangularwaveguide modes vary as a function of b/a, the ratio of the widths ofthe rectangular and square waveguides. FIG. 3 illustrates the variationof I_(mn) with b/a; i.e. the effect of varying the waveguide aspect, orwidth to height, ratio. FIG. 3 indicates that I_(mn) =0 except when m=1and n is odd. This is because of the symmetric nature of the excitationconditions. Consequently, the modes excited are only the symmetric modesEH₁₁, EH₁₃, EH₁₅ etc.

The forms of some of the lower order EH_(mn) waveguide modes are shownas electric field amplitude distributions in FIG. 4. These were obtainedby computation, and are shown as graphs (a) to (f) in quasi-threedimensional form. The coordinate axes are shown at (g). The axes x, yand z correspond to transverse vertical, transverse horizontal andlongitudinal directions in the multimode waveguide as before. The graphs(a) to (f) correspond to modes as follows:

(a): EH₁₁ ; (b): EH₂₁ ; (c): EH₃₁ ;

(d): EH₁₂ ; (e): EH₁₃ ; (f): EH₂₂.

Of these, (a), (c) and (e) are symmetric modes and (b), (d) and (f) areantisymmetric modes. To clarify this, let E(x) and E(-x) respectively bethe electrical field amplitude distributions associated respectivelywith positive and negative parts of the x axis in FIG. 1; E(x=0) is onthe z axis. Let E(y) and E(-y) be the equivalents for the y axis.

For a symmetric mode:

    E(x)=E(-x) and E(y)=E(-y)                                  (6.1)

For an antisymmetric mode, either one of or both of (6.2) and (6.3)below apply:

    E(x)=-E(-x)                                                (6.2)

    E(y)=-E(-y)                                                (6.3)

In the initial situation considered the symmetric input provides foronly symmetric modes of the multimode rectangular waveguide to beexcited.

The transverse electric field distribution in an xy plane distant z fromthe input region is E_(z) given by:

    E.sub.z =ΣA.sub.mn ·EH.sub.mn               (7)

The field intensity distribution in xy planes distant z from the inputregion is |E_(z) |², the square of the modulus or magnitude in Equation(7). |E_(z) |² has been computed as a function of distance z along therectangular waveguide for two values of b/a. In both cases, thewaveguide width (2b) was 3 mm, and its height (2a) was 1 mm in one caseand 0.5 mm in the other. This corresponds to b/a=3 and b/a=6, and thecomputation results are given graphically in FIGS. 5 and 6 respectively.FIGS. 5 and 6 give the field intensity I=|E_(z) |² as a function ofposition y across the rectangular waveguide for each of a series ofvalues of z along this waveguide. In both cases the computation wasbased on a radiation wavelength of 10.59 microns (CO₂ laser) and anactive waveguide length L of 425 mm given by Equation (1).

As illustrated in FIG. 3, when b/a=3, only the modes EH₁₁, EH₁₃, EH₁₅and EH₁₇ are excited, and these have approximate relative powers 0.32,0.33, 0.13 and 0.02 respectively. When b/a=6, the modes EH₁₁ to EH₁,13are excited with respective relative powers from 0.27 to 0.02.

In FIG. 5, an initial central maximum 80 indicates the electric fieldintensity distribution I at the input region to the rectangularwaveguide. At this point (z=0), the modes EH₁₁ to EH₁₇ give rise toelectric fields which are in phase with one another and interfereconstructively to produce the maximum 80. Moving down the length of therectangular waveguide, i.e. as z increases, the modes EH₁₁ to EH₁₇ moveout of phase with one another. This is a consequence of Equations (2)and (3), in which the phase coefficient β_(mn) and therefore also thepropagation constant γ_(mn) are dependent on the mode numbers m and n.

The spatial rates of change of these modal electrical fieldcontributions therefore differ along the z axis, i.e. axially of therectangular waveguide. This changes the form of the interference betweenmodal field contributions, and gives rise to a variety of electric fieldintensity distributions extending transversely. The distributions areindicated by curves such as 81 and 82 in xy planes at respective valuesof z. Approximately two thirds of the distance down the rectangularwaveguide, the intensity distribution is given by a curve 83 havingthree similar maxima. A distance L along the rectangular waveguide, theintensity distribution is shown as a curve 84 having two well separatedmaxima 84a and 84b. The maxima 84a and 84b are located with theircentres at the points x=0, y: -b/2, z=L and x=0, y=+b/2, z=Lrespectively. They are in phase with one another.

Turning now to FIG. 6, this shows transverse electric fielddistributions along the length of the rectangular waveguide when itscross-sectional aspect ratio b/a is 6, as previously mentioned. Asindicated in FIG. 3, the effect of increasing b/a from 3 (as in FIG. 5)to 6 (as in FIG. 6) is to reduce power coupled to rectangular waveguidemodes EH₁₁ and EH₁₃ and increase power coupled to modes EH₁₅ to EH₁,13.Since higher order modes receive more power, the degree of structure anddefinition in FIG. 6 is increased over that in FIG. 5. In FIG. 6, thefield distribution in the plane of the input region is indicated by acurve 90 with a single maximum 90a. As before, due to the modes EH₁₁ toEH₁,13 having differing β_(mn) values, the transverse intensitydistributions change with distance z along the rectangular waveguide.Curves 91 to 95 indicate locations at which there is field intensitydivision into multiple maxima of substantially equal form and magnitude.The curves 91, 92, 93, 94 and 95 have six, four, three, five and twomaxima respectively. Curve 93 in particular has three well definedmaxima 93a, 93b and 93c. The curves 91 to 95 are located at distancesfrom the waveguide input region of L/3, L/2, 2L/3, 4L/5 and Lrespectively, where L is the waveguide length as has been said. Theselengths can be expressed as 2L/6, 2L/4, 2L/3, 4L/5 and 2L/2.Accordingly, there is an inverse relationship between number of maximaand distance.

FIG. 7 shows the variation along the y axis of the phase φ of theresultant electric field for the waveguide dimensions from which FIG. 6was derived. Curves such as 100 to 105 are shown, which correspond tocurves 90 to 95 respectively. Each of the phase curves such as 101indicates the phase variation of the electric field across therectangular waveguide for a respective value of z, and corresponds to arespective intensity distribution in FIG. 6. The vertical scale of thephase representation φ is shown at 106, where an interval of 2π isindicated. The field distributions at 90 and 95 are of constant phase asindicated by straight lines 100 and 105. However, curve 103 for examplehas a central region 103a which differs in phase to its two outerregions 103b and 103c. The regions 103a to 103c give the phases ofassociated maxima 93a to 93c in FIG. 6. In consequence, the centralmaximum 93a is out of phase with the outer maxima 93b and 93c, which arein phase with one another. Since curves 100 and 105 are in phase, theyproduce reversible properties; i.e. two in-phase inputs 95a and 95bwould give rise inter alia to one output 90.

FIGS. 5, 6 and 7 relate to specific values of b/a. More generally, forthe situation initially considered, only EH_(1n) modes are excitedbecause of the EH₁₁ symmetry of the excitation from the input radiationbeam. At the rectangular waveguide input region, the phase is constant.For the case involving arbitrary b/a values, using Equation (3) thephase coefficient β_(1p) of mode EH_(1p) is given by: ##EQU6## and thephase coefficient β_(1q) of mode EH_(1q) is given by: ##EQU7##

By subtraction of Equation (9) from Equation (8) and rearranging, thephase difference between modes EH_(1p) and EH_(1q) at guide length z isX_(z) given by: ##EQU8##

If the condition is imposed that a 2π phase difference exist between themodes, Equation (10) becomes: ##EQU9## and the propagation distance z(say z₂π) in Equation (11) in rectangular waveguide that gives rise to a2π phase difference between modes EH_(1p) and EH_(1q) is given by:##EQU10##

For the case of the EH₁₁ and EH_(1n) modes (i.e. the fundamental andn^(th) highest order odd mode) z₂π is given by ##EQU11##

Combining Equations (2) and (13): ##EQU12##

With n=3,5,7,9,11 . . . 16L/n² -1) z₂π is 2L, 2L/3, L/3, L/5, 2L/15 . ..

As fractions of a propagation distance 2L in rectangular waveguide whichresults in the EH₁₁ and EH₁₃ modes being back in phase, the relativelength ratios are 1, 1/3, 1/6, 1/10, 1/15 etc. This shows that there isa harmonic relationship between the EH_(1n) modes of the rectangularguide. Equation (4) shows that the propagation distance z₂π which givesrise to a 2π phase shift between the fundamental EH₁₁ mode and the nexthighest order EH₁₃ mode also gives rise to a 2π phase shift between thefundamental and all other EH_(1n) modes (n odd). This results inreproduction of any symmetric input electric field after a distance z₂π,provided that there is only excitation of odd EH_(1n) modes. A symmetricinput field is also produced periodically at distances of tz₂π, where"t" is an integer number if there is sufficient length of rectangularwaveguide available.

Equations (11) to (14) may be rewritten to determine z.sub.π, thepropagation distance in rectangular waveguide over which an intermodephase change of π is introduced. By inspection of these equations, it isseen that: ##EQU13##

L and 2L are the waveguide lengths over which z.sub.π and z₂π areintroduced, and L=2b² /γ from Equation (1). In consequence, z.sub.π andz₂π are both proportional to b², and may be arranged to occur atprearranged distances along a rectangular waveguide by suitable choiceof the waveguide width.

Returning to the mixer 10, of FIGS. 1 and 2, the appropriate modestructure within the waveguide 14 is the reverse of that illustrated inFIGS. 5 and 6. That is there are two fundamental EH₁₁ mode inputs, alength L of rectangular waveguide, and a single centrally positioneddetector or output means. However, as previously mentioned, when thereare two or more input radiation beams the relative phases of the inputsare important, and must be selected appropriately for the desiredoutput.

Referring now to FIGS. 8 and 9, the variation of electric fieldintensity distribution I with distance along a multimode waveguide isillustrated. These drawings relate to two equal intensity inputsin-phase and in anti-phase respectively. In FIG. 8, two initial maxima110 and 112 indicate the electric field intensity distribution I at z=L.They are positioned on the y-axis at -b/2 and +b/2 respectively.

The relative phases of the maxima 110 and 112 are indicated at 114. Asfor beam splitting, modal dispersion occurs in the waveguide and after alength L (i.e. at z=L) a single maxima 116 is produced, positioned onthe y-axis at y=0. In FIG. 9, two initial maxima 120 and 122 indicatethe electric field intensity distribution I at z=0. They are positionedat y=-b/2 and y=+b/2 respectively. The relative phases of the maxima 120and 122 are indicated at 124. Again modal dispersion occurs in thewaveguide, but after a length L (i.e. at z=L) two maxima 126 and 128 areproduced. They are positioned on the y-axis at -b/2 and +b/2respectively. Thus the input electric field intensity distribution hasbeen reproduced after a length L of waveguide. For phase conditionsintermediate the two extremes illustrated in FIGS. 8 and 9, three outputmaxima will be produced at z=L. The respective amplitudes will bedependent on the relative phases of the inputs.

The output effects produced by the relative phases of a plurality ofinputs to the rectangular waveguide are a result of the modes excited.This is discussed below with reference to FIGS. 10 and 11 whichgraphically illustrate relative modal amplitudes for the three lowestodd and even EH_(mn) modes respectively, as an input waveguide bearing afundamental mode beam is offset from the centre of the rectangularwaveguide input region. In the device 10 the two inputs are locatedwithin the input region 22 at y=±b/2. As can be seen from FIG. 10 theodd modes, EH₁₁, EH₁₃ and EH₁₅ are excited with identical amplitudes byinputs in these two positions. However, as can be seen from FIG. 11 theeven modes, EH₁₂, EH₁₄ and EH₁₆ are excited with amplitudes of identicalmagnitude but opposite sign by two such inputs. Therefore when the twoinputs are in phase with each other the excitations of the odd modes sumto produce twice the amplitude of a single input at -b/2 or +b/2 whilstthe even modes cancel each other out. As was shown earlier, excitationof only odd modes (n=1,3,5 etc) leads to the two inputs summing to forma single maximum at the out region 24. When the two inputs are inantiphase with each other the odd modes cancel out and the even modessum to produce twice the amplitude of a single input at -b/2 or +b/2.Again, as was shown earlier, this input condition produces two maxima atthe output region 24.

The mixer 10 may be designed for operation with radiation from a CO₂laser of wavelength 10.59 μm. Its dimensions may be 2a=0.6 mm, 2b=1.2 mmand L=2b² /λ=106 mm. The setting of b=2a indicates the minimum width andhence length for which the mixer 10 may be constructed. However, withb=2a the three output maxima produced for most input phase conditionsare not fully resolved. The inner tails of the laterally positionedmaxima overlap with the tails of the central maximum. Thus a detector 34of width 2a will never receive zero intensity, but the intensity willvary with the beat frequency. The detector 34 may be narrower than 2a ifrequired. Alternatively the dimensions of the mixer 10 may be 2a=0.6 mm,2b=1.8 mm and L=2b² /λ=153 mm. In this case b=3a and the three outputmaxima will overlap less and therefore be better resolved. However toachieve full resolution of the three maxima a rectangular waveguide withb=4a is necessary, and this requires L=2b² /λ=272 mm if 2a=0.6 mm.

The waveguides 14, 26, 28 of the device 10 may be constructed, for useat 10.59 μm of materials other than alumina, e.g. BeO, Si, Macor ormetal. Furthermore, the square waveguides 26 and 28 may be replaced byother forms of waveguide. For instance, a device of the invention mayincorporate square section waveguides of side 2a with sides at 45° tothe x and y axes, or elliptical guides with the major and minor axesparallel to the x and y axes. However the square of the waveguide depthshould be an integral multiple of the product of multimode waveguidelength and wavelength in that waveguide. Other shapes such as diamond oroctagonal guides may also be used. In addition optical fibres may beused.

The mixer 10 may have various additional features. It may beadvantageous for the remaining area of the output region 24, notoccupied by the detector 34, to be made from an absorbent material, orbear an antireflective coating. This would prevent radiation reflectingback into the rectangular waveguide 14 and interfering with the desiredmode structure.

Referring now to FIG. 12 an alternative embodiment of the invention isillustrated schematically. It consists of a mixer 130. The mixer 130 0operates in a very similar manner to the mixer 10 and therefore thefollowing description concentrates on areas of difference. Parts commonto the mixer 10 of FIG. 1 are like referenced with the addition of anasterisk superscript (*). The essential difference between mixers 10 and130 is the presence of two additional detectors 132 and 134, andassociated circuitry (not shown). The two additional detectors 132, 134are linked together such that their output signals are combined.

The additional detectors 132, 134 are located in the z=L plane withtheir centres at x=0, y=±b/2. Thus in operation as a heterodyne mixer acentral output maximum when produced would be incident on the detector34* whilst lateral output maxima would be incident on the additionaldetectors 132, 134. As the intensity incident on the central detector34* falls, that incident on the lateral detectors 132, 134 rises. Themixer 130 does not obtain any additional information over that obtainedby the mixer 10 but, depending on operating conditions, may provide animproved signal to noise ratio.

As discussed above the resolution of the three output maxima depends onthe width of the rectangular waveguide 14*. If it is desired that themaxima are fully resolved and the detectors 34*, 132, 134 are 2a widethen b must be set at 4a and the length will be 32a² /λ. However if thelength of the mixer 130 is required to be shorter the detectors 34*,132, 134 may be narrower. For instance if b=5a, and the detectors 34*,132 and 134 are 1.2a wide then the mixer 130 will be L=12.5a² /λ longand have reasonable performance.

Referring now to FIG. 13, a further two input mixer of the invention isillustrated schematically. The mixer is indicated generally by 170. Itincorporates a rectangular waveguide 172 of width 2b, height 2a andlength L=8b² /λ. The width 2b must in general be equal to or greaterthan 4a, and in this case 2b=4a. The waveguide 172 has input and outputregions 174, 176 respectively at its mutually opposite longitudinalends. As for previously described embodiments Cartesian co-ordinateswill be employed to describe positions within the device 170, the axesand origin are similarly defined. Two input square waveguides 178, 180are connected to the input region 174 such that input radiation beamsprovided by them are centred at x=0, y=-b/2 and x=0, y=+b/2respectively. Two detectors 182, 184 are located in the output region176 centred at x=0, y=-b/2 and x=0, y=+b/2.

The two input square waveguides 178, 180 each provide a fundamental modeinput. As described earlier, these may be a local oscillator signal anda Doppler shifted received signal respectively. When the two inputs areeither in phase or in antiphase with one another, two substantiallyequal intensity maxima are produced in the output region 176, centred onx=0, y=±b/2. When the received signal in waveguide 180 is 90° ahead ofthe local oscillator signal in waveguide 178 then a single maximum isproduced in the output region centred on x=0, y=+b/2. When the receivedsignal is 270° ahead of the local oscillator signal then a singlemaximum centred on x=0, y=-b/2 is produced. Thus, the intensity incidenton each of the detectors 182, 184 varies at the beat or differencefrequency.

Referring to Table 1 the relative dimensions of the various embodimentsdescribed, of two input mixers of the invention, are summarised. It canbe seen from Table 1 that a compromise has to be reached between thelength of the device and the resolution of the output maxima. In manyapplications the length will be the more important criterion. There mayhowever be manufacturing processes which make relatively straightforwardthe production of devices each having a rectangular waveguide the samewidth as the sum of the widths of associated input waveguides.

                  TABLE 1                                                         ______________________________________                                        Relative Dimensions for Various Embodiments of                                Two Input Mixers of the Invention                                             Rectangular Waveguide                                                         Width                                                                         (2b)     Length     Inputs     Outputs                                        ______________________________________                                        4a       8a.sup.2 /λ                                                                       y = ±b/2                                                                              y = 0, ±b/2                                          (2b.sup.2 /λ)  partially resolved                             6a       18a.sup.2 /λ                                                                      y = ±b/2                                                                              y = 0, ±b/2                                          (2b.sup.2 /λ)  partially resolved                             8a       32a.sup.2 /λ                                                                      y = ±b/2                                                                              y = 0, ±b/2                                          (2b.sup.2 /λ)  fully resolved                                 4a       32a.sup.2 /λ                                                                      y = ±b/2                                                                              y = 0, ±b/2                                          (8b.sup.2 /λ)  fully resolved                                 ______________________________________                                    

A further alternative to the embodiments described above will now bedescribed. It incorporates a multimode waveguide of width 2b greaterthan 4a, but with two input waveguides positioned at y=±(b-a). That isthe input waveguides are positioned adjacent to the sides of the inputregion of the multimode waveguide, with a gap therebetween. This devicestructure will lead to some distortion of the electric field structuredescribed earlier, in particular of the central output maximum. Howeverit will provide for zero intensity at the centre of the output regionwhen the two inputs are in phase. The distortion may thus be acceptableif the result is improved discrimination at the detectors. Suitablerelative dimensions may be 2b=4.5a and L=10.125a² /λ.

Embodiments of the invention may be constructed with alternatives todetectors located in the rectangular waveguide output region. Forinstance output waveguides may be positioned to accept the maxima inplace of the detectors, with detectors located within the outputwaveguides remote from the output region.

The invention is not limited to mixers for use in heterodyne detection.There are many applications where it is desired to combine radiationbeams prior to detection for which embodiments of the invention maybeused. In addition the invention is not limited to mixing devices withtwo inputs.

The invention is also not limited to use of hollow core opticalwaveguides for use of wavelengths of 10.59 μm. Provided the devices areconstructed from appropriate materials for the wavelength of operation,they may be constructed for use over a wide range of wavelengths. Forinstance they may be constructed, using semi-conductor layer technology,from GaAs and AlGaAs for use with radiation from Nd-YAG lasers (freespace wavelength λ=1.064 μm).

Referring now to FIG. 14, a laser radar system incorporating a device ofthe invention, in the form of a heterodyne mixer, is illustratedschematically. The system is indicated generally by 200. It incorporatesa laser radiation source 202, a beamsplitter 204, an acousto-opticmodulator 205 and a mixing device of the invention 206 similar to thedevice 10 of FIG. 1. It also incorporates optical waveguides 208, 210,212, 214 and 215 directing radiation between optical components and toand from a target zone (not shown). The mixing device 206 incorporates adetector 216 and associated circuitry 218.

The system 200 operates as follows. Radiation emitted by the laser 202passes down the waveguide 208 to the beamsplitter 204. It is split intofirst and second beams. The first beam passes down the waveguide 210 andbecomes an output beam to the target zone. The second beam passes alongthe waveguide 212 to the acousto-optic modulator 205 where it undergoesa small frequency shift. The frequency shifted beam passes to the mixingdevice 206 and is employed as a local oscillator signal. Radiationreflected or scattered from the scene is received by the waveguide 214,and passes to the mixing device 206. It forms a received signalincluding Doppler shifted components reflected from moving objects orparticles. The Doppler shift may be an increase or decrease infrequency.

The mixing device 206 operates as previously described for the device10. Radiation passing to the mixing device 206 along waveguides 212 and214 is mixed and the intensity incident on the detector 216 varies withdifference frequency of the local oscillator signal and the receivedsignal. An output electrical signal is provided, by the associatedcircuitry 218, at 220. The purpose of the acousto-optic modulator 205 toavoid the loss of decreased frequency components after subtraction ofthe local oscillator frequency, and which would otherwise correspond tonegative frequencies; is a source frequency to which becomes downshiftedby a Doppler frequency shift f_(D) becomes a frequency f_(o) -f_(D).Subtraction of the local oscillator frequency (also f_(o)) apparentlyproduces -f_(D), which does not exist. To avoid this, subtracting amodulation frequency f_(A) from the source frequency produces a localoscillator frequency f_(o) -f_(A). A negative Doppler shift then gives afrequency f_(A) -f_(D), which can be arranged to remain positive.

Referring now to FIG. 15, there is shown a further embodiment of amixing device of the invention indicated generally by 230. Itincorporates two square section fundamental mode input waveguides 232 ofside 2a, these being connected to a rectangular multimode waveguide 234of cross-section 2a by 2b and length 4b² /λ. As before, λ is thewavelength of radiation in the rectangular waveguide 234 for which thedevice 230 is designed. The rectangular waveguide 234 has two detectors236 with associated output lines 238.

The input waveguides 232 have central axes 240 offset by b/2 onrespective sides of a rectangular waveguide central longitudinal axis242. The detectors 238 are centred on respective input waveguide axes240.

The device 230 operates as follows. Radiation beams (not shown) areinput to the input waveguides 232 and propagate therein in fundamentalmode. On reaching the rectangular waveguide 234 they undergo modaldispersion. If the two contributions of radiation reaching therectangular waveguide are in phase with one another, the detectors 236receive respective radiation intensity maxima of like magnitude centredon axes 240. If however one input contribution leads the other in phaseby 90°, then that detector 236 which is aligned with the input waveguide232 providing that contribution receives 85% of the radiation intensityreaching the right hand end of the rectangular waveguide 234 and theother detector receives 15%. Variation in the input phase as a functionof time produces a variation in each detector signal between 15% and 85%of the sum of the signals. Unlike the device 170 of FIG. 13, thetheoretical modulation depth is only 70% instead of 100%, but againstthis the device 230 is more compact because it has a rectangularwaveguide 234 half the length of the equivalent 172 for the device 170.

We claim:
 1. An optical mixing device including:(a) a multimode mixerwaveguide having an input region and an output region, and (b) two inputwaveguides, operational in a fundamental mode and coupled to the inputregion, said two waveguides comprising a means for providing twofundamental mode radiation beams input to the mixer waveguide, and (c)detecting means for receiving a mixed radiation output transmitted bythe mixer waveguide to the output region,wherein the dimensions of themixer waveguide, and the positions and spatial characteristics of theinput radiation beams in combination comprise a means for modallydispersing both input radiation beams within the mixer waveguide and formixing both input radiation beams to provide a mixed radiation output,said detecting means comprising a means for detecting said mixedradiation output and for providing an output variable in response torelative phase changes of said input radiation beams.
 2. An opticalmixing device according to claim 1 wherein the detecting means islocated centrally of the mixer waveguide transverse cross-section.
 3. Anoptical mixing device according to claim 1 wherein the mixer waveguideis of rectangular cross-section.
 4. An optical mixing deviceincluding:(a) a multimode mixer waveguide having an input region and anoutput region, and (b) two input waveguides arranged for fundamentalmode operation are coupled to the input region and are arranged toprovide two radiation beams input to the mixer waveguide, and (c)detecting means are arranged to receive radiation transmitted by themixer waveguide to the output region,and wherein the dimensions of themixer waveguide, and the positions and spatial characteristics of theinput radiation beams are in combination arranged to provide for modaldispersion in the mixer waveguide giving rise to input radiation mixingin the output region and mixed radiation detection by the detectingmeans, wherein the mixer waveguide is of rectangular cross-section, andwherein a first detecting means is located centrally within the outputregion and second and third detecting means are located centrally ofrespective halves of the mixer waveguide transverse cross-section at theoutput region.
 5. An optical mixing device according to claim 4 whereinthe mixer waveguide has a greater cross-sectional side of length 2b, theinput waveguides comprise a means for providing input radiation with awavelength λ measured within the mixer waveguide and the mixer waveguideis of length L of 2b² /λ.
 6. An optical mixing device including:(a) amultimode mixer waveguide having an input region and an output region,and (b) two input waveguides arranged for fundamental mode operation arecoupled to the input region and are arranged to provide two radiationbeams input to the mixer waveguide, and (c) detecting means are arrangedto receive radiation transmitted by the mixer waveguide to the outputregion,and wherein the dimensions of the mixer waveguide, and thepositions and spatial characteristics of the input radiation beams arein combination arranged to provide for modal dispersion in the mixerwaveguide giving rise to input radiation mixing in the output region andmixed radiation detection by the detecting means, wherein the mixerwaveguide is of rectangular cross-section, and wherein said detectingmeans includes first and second detecting means, said first and seconddetecting means comprising a means for detecting spatial variations inintensity of the mixed radiation within the output region.
 7. An opticalmixing device according to claim 6 wherein the first and seconddetecting means are located centrally of respective halves of the mixerwaveguide transverse cross-sectional at the output region.
 8. An opticalmixing device according to claim 7 wherein the mixer waveguide has agreater cross-sectional side of length 2b, the input waveguides comprisea means for providing input radiation with a wavelength λ measuredwithin the mixer waveguide and the mixer waveguide is of length L of 8b²/λ or 4b² λ.
 9. An optical mixing device including:(a) a multimode mixerwaveguide having an input region and an output region, and (b) two inputwaveguides arranged for fundamental mode operation are coupled to theinput region and are arranged to provide two radiation beams input tothe mixer waveguide, and (c) detecting means are arranged to receiveradiation transmitted by the mixer waveguide to the output region,andwherein the dimensions of the mixer waveguide, and the positions andspatial characteristics of the input radiation beams are in combinationarranged to provide for modal dispersion in the mixer waveguide givingrise to input radiation mixing in the output region and mixed radiationdetection by the detecting means, wherein the mixer waveguide is formedas a hollow within solid dielectric material.
 10. An optical mixingdevice according to claim 9 wherein the material is alumina.
 11. Anoptical mixing device including:(a) a multimode mixer waveguide havingan input region and an output region, and (b) two input waveguides,operational in a fundamental mode and coupled to the input region, saidtwo input waveguides comprising a means for providing two fundamentalmode radiation beams input to the mixer waveguide, and (c) detectingmeans for receiving radiation transmitted by the mixer waveguide to theoutput region,wherein the dimensions of the mixer waveguide, and thepositions and spatial characteristics of the input radiation beams incombination comprise a means for providing modal dispersion in the mixerwaveguide giving rise to input radiation mixing in the output region andmixed radiation detection by the detecting means, wherein the mixerwaveguide is formed as a ridge waveguide upstanding from a substrate.12. An optical mixing device according to claim 11 wherein it is formedof layers of materials of one of a ternary and quaternary semiconductormaterial system.
 13. An optical mixing device according to claim 12wherein the material system is Al_(x) Ga_(1-x) As.
 14. An optical mixingdevice including:(a) a multimode mixer waveguide having an input regionand an output region, and (b) two input waveguides arranged forfundamentals mode operation are coupled to the input region and arearranged to provide two radiation beams input to the mixer waveguide,and (c) detecting means are arranged to receive radiation transmitted bythe mixer waveguide to the output region.and wherein the dimensions ofthe mixer waveguide, and the positions and spatial characteristics ofthe input radiation beams are in combination arranged to provide formodal dispersion in the mixer waveguide giving rise to input radiationmixing in the output region and mixed radiation detection by thedetecting means, wherein the input waveguides are of squarecross-section.
 15. An optical mixing device including:(a) a multimodemixer waveguide having an input region and an output region, and (b) twoinput waveguides arranged for fundamental mode operation are coupled tothe input region and are arranged to provide two radiation beams inputto the mixer waveguide, and (c) detecting means are arranged to receiveradiation transmitted by the mixer waveguide to the output region,andwherein the dimensions of the mixer waveguide, and the positions andspatial characteristics of the input radiation beams are in combinationarranged to provide for modal dispersion in the mixer waveguide givingrise to input radiation mixing in the output region and mixed radiationdetection by the detecting means, wherein one of the input waveguides iscoupled to a coherent radiation source and the two input radiation beamsare a local oscillator signal derived from the said source and a returnsignal received from a target zone by virtue of one of reflection andscattering of an output beam from the said source, the device comprisinga heterodyne mixer.